New on the Network Tools page: a VLAN / QinQ Tag Overhead Calculator. It exists because of the same problem that led to the MTU / Encapsulation Overhead Calculator — vendors don’t agree on what a config knob actually means, and it costs you an afternoon before you find that out.

The history behind this one

The MTU tool exists because mtu isn’t the same number on every platform. Classic Cisco IOS treats it as the L3 payload size. Junos and IOS-XR fold the 14-byte Ethernet header into it. Same command, same-looking number, two different frames on the wire — and the failure mode is never an error message, it’s just silent fragmentation or a black-holed jumbo-frame flow that only shows up under load.